上方基坑开挖引起下卧隧道纵向变形分析方法

Approach for evaluating longitudinal deformation of underlying tunnels due to excavation of upper foundation pit

上方基坑开挖引起下卧隧道纵向响应的分析方法大多将基坑开挖处理为基底简单卸荷作用,采用基底卸荷引起的隧道轴线处自由土体的附加应力作为输入条件,视之为隧道所受到的荷载。然而,这种力输入条件并非隧道受到的真实荷载,导致得不到合理的隧道响应。为此,首先从理论上分析当前的力输入条件的不足以及位移输入条件较为合理的原因,并对两种输入条件下Winkler地基模量进行区分,指出位移输入条件和力输入条件应分别采用被动和主动Winkler地基模量。其次,通过对比有限元和Winkler方法得到的隧道响应,验证了位移输入条件的合理性,而当前的力输入条件会低估隧道的响应。最后,鉴于力输入方法的工程实用性,提出利用荷载放大系数调整输入荷载的大小,以获得合理的隧道响应。利用大量的有限元算例进行研究,确定了荷载放大系数的简化计算公式,通过与工程案例的实测结果对比,验证了所提出方法的可行性。

For most of the analysis methods for the longitudinal response of underlying tunnels,excavation of the foundation pits is treated as a simple unloading of the base and the additional stress of the greenfield soil at the tunnel axis is taken as the input condition.However,this force input condition is not the real load to which the tunnel is subjected,resulting in an unreasonable tunnel response.To this end,the reasons why the current force input condition is insufficient and the displacement input condition is more reasonable are firstly analyzed theoretically,and the Winkler subgrade moduli of the two input conditions are distinguished.It is proposed that the passive and active Winkler subgrade moduli should be used for the displacement and pressure input conditions,respectively.Secondly,by comparing the tunnel response obtained by the finite element model and the Winkler methods,it is verified that the displacement input condition is reasonable,while the current force input condition would underestimate the tunnel response.Finally,given the engineering practicality of the force-based method,it is proposed to adjust the magnitude of the force input condition by using the load amplification factor to obtain a reasonable tunnel response.Using a large number of finite element calculations for parametric studies,a simplified formulation for the load amplification factor is proposed.The feasibility of the proposed method is verified by comparing with the field measurements of a case study.